4,4' bisphenol sulfone (4,4' dihydroxydiphenyl sulfone) has found particular utility as a monomer in the production of high molecular weight polymers such as polyphenylsulfone resins. For such use, however, 4,4' bisphenol sulfone must be substantially free of its 2,4' bisphenol sulfone isomer and be of high purity in order to yield a quality polymer product. Polymers made from 4,4' bisphenol sulfone having an appreciable amount of 2,4' isomer generally do not reach as high a molecular weight and do not exhibit mechanical properties as excellent as polymers made from purified 4,4' bisphenol sulfone. As little as 2 percent impurity can cause the 4,4' bisphenol sulfone to be unsatisfactory as a monomer for certain polymerizations.
Commercial processes generally have not been able to synthesize 4,4' bisphenol sulfone of sufficient purity for its general application as a polymerization monomer. Almost invariably 4,4' bisphenol sulfone is formed in admixture with undesirably large amounts of its 2,4' isomer. Depending on the method of its synthesis, 4,4' bisphenol sulfone also may contain minor amounts of a variety of other impurities such as phenol, phenol sulfonic acid, quinone-type coloring agents, resins and tars.
Accordingly, the literature is replete with descriptions of proposed methods for recovering 4,4' bisphenol sulfone from mixtures of 4,4' and 2,4' bisphenol sulfone. Proposed recovery methods include various processes using organic solvents to extract the 2,4' isomer and recover the undissolved 4,4' isomer from isomer mixtures of bisphenol sulfone. For example, there have been described solvent extraction methods using ethers (U.S. Pat. No. 2,833,828), S-tetrachloroethane (U.S. Pat. No. 3,065,274) and esters (U.S. Pat. No. 3,551,501). These methods, however, generally require a high temperature solvent extraction step, followed by a solvent wash step, and then a recrystallization step to recover purified 4,4' bisphenol sulfone from the bisphenol sulfone mixture. This succession of steps inevitably results in substantial loss of 4,4' bisphenol sulfone, as well as solvent, thereby rendering the methods commercially unacceptable.
Recovery of 4,4' bisphenol sulfone by recrystallization from water also has been often proposed. Due to the low solubility of bisphenol sulfone in water, the described methods employ heated solutions of alkali (U.S. Pat. No. 4,162,270) or convert bisphenol sulfone to an alkali metal salt (U.S. Pat. No. 3,297,666) in order to more readily achieve an aqueous solution to which is added activated carbon that adsorbs impurities. After filtration, the aqueous solution is neutralized or acidified and the bisphenol sulfone recrystallizes. These recrystallization methods, however, are only effective in recovering 4,4' bisphenol sulfone product free from by-product coloring agents and resinous impurities, as substantial amounts of 2,4' isomer precipitate along with the 4,4' isomer.
Other aqueous recrystallization methods that have been proposed for recovering 4,4' bisphenol sulfone are described as involving the formation and heat dissolution of a calcium complex (U.S. Pat. No. 2,392,137) or an alkali-metal complex (U.S. Pat. No. 3,277,183) which precipitate upon cooling of the solution. These methods, though they are said to be effective in removing some 2,4' isomer, nevertheless coprecipitate 2,4' bisphenol sulfone in amounts such that successive recrystallizations are required to recover 4,4' bisphenol sulfone of high purity. Again the succession of steps results in unacceptable product loss.